There are at least three major criteria in formulating an oil gel for cables. The oil gel must possess some degree of slump resistance at high service temperatures. Additionally, the gel needs to be strippable so that installers and repairmen can make electrical splices easily in the field. Also, the oil gel should have a good working viscosity in the melt so it can be easily pumped into the cable.
Filling compounds used to prevent the ingress of water into telecommunications cable must have processing characteristics which allow the material to penetrate and fill the voids between densely packed insulated conductors. Application viscosity is critical and the ability to adjust the viscosity by temperature is limited by potential damage to the insulation on the copper conductors. Once the cable is filled, the filling compound must not flow out at temperatures of up to 80.degree. C., must withstand significant heads of water, should have good craft handling characteristics, must be compatible with other components in the cable system such as splice encapsulants, and should not significantly add to the stiffness of the cable.
Lower molecular weight polymers like KRATON.RTM. G1650, G1726, and G1652 polymers are used in the cable filling industry. KRATON.RTM. G1650 and G1652 polymers possess good strippability benefits (as measured by oil gel tear strengths), and are of low enough viscosity to pump into the cable and fill all the crevices between bundles of wire in cables. The major problem with KRATON.RTM. G1650 and G1652 polymers is that these polymers in oil gel formulations do not perform well at high service temperatures. This is due to the relatively low molecular weight of the polystyrene endblocks.
Higher molecular weight versions like KRATON.RTM. G 1651 and G1654 polymers show promise for excellent service temperature performance. The large styrene endblocks are much more resistant to flow (and loss of elasticity), thus giving high service temperature performance. The large endblocks also help produce oil gels which, under some conditions, can be difficult to strip (high tear resistance), and prevent flow at service temperatures. Unfortunately, oil gels based on KRATON.RTM. G1651 and G1654 polymers have poor adhesion and their viscosity is too high at application temperatures to allow the gel to flow properly between the bundles of wires in a cable. Therefore, KRATON.RTM. G1651 and G1654 polymers is not used extensively in cable filling applications.
This invention provides the advantages of both low molecular weight polymers and higher molecular weight polymers while minimizing their disadvantages. Using the high vinyl content polymers of the present invention in an oil gel application allows oil gel formulators to manufacture gels with high service temperature properties in a strippable and pumpable form. The present wisdom in this art suggests that high service temperatures and a reduction of application viscosity may be mutually exclusive for oil gels. This invention provides a novel way to produce compositions which exhibit both of these characteristics. Using high vinyl content polymers as opposed to the lower vinyl content polymers discussed above fortifies the viscosity/concentration relationship of the polymers, i.e. lower viscosity with other properties remaining about the same.